James, what I am thinking is to go with a Eurocard style frame, which provides much higher density connectors for complicated modules like you are talking about and for tube modules having high current requirements, you can then use multiple pins to pass req'd current.
Then you could have a simple 500 Edac to DIN41612 adaptor so you can mix and match 500 and non-500 type modules.
Also if you need more real estate for knobs and switches, you could then go with a double width module, or like in the Eurocard format where they have 3, 4, 5, 6, 7, 8, 10, 12 ..... 84HP width module options.
Just a thought
Michael